Wafer structure for electronic integrated circuit manufacturing

ABSTRACT

A bonded wafer structure having a handle wafer, a device wafer, and an interface region with an abrupt transition between the conductivity profile of the device wafer and the handle wafer is used for making semiconductor devices. The improved doping profile of the bonded wafer structure is well suited for use in the manufacture of integrated circuits. The bonded wafer structure is especially suited for making radiation-hardened integrated circuits.

RELATED CASE

The present application claims priority from, and is a divisional of,U.S. patent application Ser. No. 13/218,308 filed on Aug. 25, 2011 whichis entitled “WAFER STRUCTURE FOR ELECTRONIC INTEGRATED CIRCUITMANUFACTURING” which is hereby incorporated by reference in its entiretyfor all purposes as if fully set forth herein.

FIELD OF THE INVENTION

The present invention relates to wafer structures used for themanufacture of electronic integrated circuits, and more particularly tosemiconductor integrated circuits, including complementarymetal-oxide-silicon (CMOS) integrated circuits.

BACKGROUND OF THE INVENTION

It is well known that ionizing radiation (such as gamma rays, x-rays,electrons, neutrons, and protons) cause degradation of electronicintegrated circuits, especially semiconductor integrated circuits.Several different radiation effects have been observed since the 1970's,and have been studied for several decades following the firstobservations. One effect of ionizing radiation on metal oxidesemiconductor (MOS) integrated circuits is a cumulative degradation ofthe circuit due to trapping of radiation induced charges in thedielectric layers and gate regions and at the silicon:silicon-dioxideinterface of MOS devices. The deleterious effects include an increase inleakage currents and threshold voltage shifts due to the trapped holes(and/or interface traps, border traps, or other similar trapped chargestates). A review of such total ionizing dose (TID) effects has beenprovided by Barnaby (H. J. Barnaby, “Total-Ionizing-Dose Effects inModern CMOS Technologies”, IEEE Transactions on Nuclear Science Volume56 Number 6, pp 3103-3121, 2006).

Another class of ionizing radiation damage is called single eventeffects (SEE's). SEE's result from a transient deposition of charge inthe integrated circuit due to a single heavy ion, a single proton, orother single particle. An SEE may cause an upset in the value of amemory bit, a transient analog signal, an electrical latch-up, a gatedielectric rupture, or many other effects. Some such SEE's causeirreversible damage to the integrated circuit, such as in single eventlatch-up (SEL). Other SEE events only cause loss of data, and may berecovered by re-writing the affected memory location with the correctdata, as in single event upset (SEU).

Process independent approaches utilizing device and circuit designtechniques have also been utilized, but these methods resulted ininferior electrical performance and increased circuit size. All of thesehardening approaches are well known in the art and have proved veryeffective in mitigating and/or preventing total ionizing dose changes inMOS integrated circuits specifically designed or re-designed for use inradiation environments. What is desired, therefore, is an integratedcircuit wafer structure and corresponding method that is radiationhardened, yet maintains good electrical performance without the penaltyof increased circuit size.

SUMMARY OF THE INVENTION

According to embodiments of the present invention, a family ofengineered wafer structures is compatible for use as substrates formanufacturing existing commercial electronic integrated circuits (IC's),or for manufacturing new radiation hardened integrated circuits withdesign rules that allow for better electrical performance and decreasedcircuit size.

According to embodiments of the present invention, hardening of suchexisting IC's against both total ionizing dose (TID) radiation effects(including, neutrons and the secondary particles generated by theneutrons) and single event effects (SEE) is made possible by bothpinning the surface potential of the affected interface regions in theintegrated circuit, by limiting the generation of electron hole pairsproduced in sensitive regions of the integrated circuit due tobombardment of the IC with heavy ions, gamma rays, x-rays, electrons,protons, or other types of ionizing radiation, and by reducing theminority carrier lifetime in the base regions of the parasitic siliconcontrolled rectifier (SCR) structures that are inherent in modern CMOSIC's.

Further, the embodiments of the present invention can be combined insuch a way as to provide both an improved substrate for manufacturingexisting circuits, or to provide hardening of newly designed circuitseither alone or in combination with existing designs.

Embodiments of the present invention also provide improved hardening offully completed integrated circuits via high energy monoenergeticneutron bombardment. Monoenergetic neutron bombardment allows forcreation of the defects to reduce the minority carrier lifetime in thebase regions of the parasitic SCR structures inherent in modern CMOSIC's. Previous work on neutron bombardment of existing ICs provedlargely ineffective due to the concomitant large amount of totalionizing radiation delivered along with the neutron irradiation. A newfamily of neutron generators based on a deuterium-tritium (D-T) ordeuterium-deuterium (D-D) reaction are capable of delivering orders ofmagnitude more neutron irradiation without a significant increase intotal ionizing dose.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of a directed wafer bonding (DWB) method showinga silicon-to-silicon direct bonded flow according to the presentinvention;

FIG. 2 is a graph of a DWB doping profile according to the presentinvention, showing the doping density of a handle wafer, the dopingdensity of a device wafer, and the abrupt transition between the twodoping densities in an interface region;

FIG. 3 is an illustration of a bonded wafer according to the presentinvention, and the corresponding perpendicular and parallel conductivityprofiles thereof;

FIG. 4 is an illustration of a bonded wafer according to the presentinvention including an integrated circuit fabricated in the devicewafer;

FIG. 5 is an illustration of a wafer structure according to the presentinvention containing multiple interface regions;

FIG. 6 is an illustration of a wafer structure according to the presentinvention wherein the device region is not co-extensive with the handlewafer;

FIG. 7 is a plot of neutron flux (14 MeV), for a total source output of1×10¹¹ n/s, according to the present invention; and

FIG. 8 is a schematic diagram of a neutron source test cell according tothe present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a flow chart 10 of a directed wafer bonding(DWB) method shows a silicon-to-silicon directed bonded epitaxial flowaccording to the present invention. The method shown in FIG. 1 issuitable for fabricating photodiodes, but other semiconductor structurescan also be fabricated. In general, a silicon device wafer 12 is bondedto a silicon handle wafer 14. The silicon wafers are typically eightinch wafers each having a thickness of about 725 microns. Other wafersizes can obviously be used. The wafers each have separate dopingprofiles, and can be the same or a different polarity type as will beexplained in further detail below. The bonding operation can beaccomplished in a variety of different ways. For example a bonding agentsuch as nitric acid mixed with hydrogen peroxide can be used, followedby a thermal step of baking at 200 degrees to 1,200 degrees Celsius for1 to 4 hours. Alternatively, a bonding agent such as a chemical vapordeposited (CVD) oxide film on at least one of the two wafers beingbonded can also be used, followed by bonding at a temperature from 200to 1,300 degrees Celsius, further followed by thinning the bonded waferstructure to the desired final thickness, further followed by a thermalstep of baking at 600 to 1,300 degrees for 1 second to 10 hours in areducing atmosphere containing hydrogen and nitrogen to eliminate theinsulating material at the wafer bond interface. Once the wafers arebonded together, they are ground and polished according to known methodsin the art. The device wafer is ground down to a thickness of about oneto ten microns. The exact depth can change, and can in fact be greaterthan ten microns if desired. The handle wafer need not be ground down,or only ground down slightly so that the total bonded wafer thickness isabout 725 microns for a 200 mm diameter wafer, such that the combinationwafer is readily processed in conventional semiconductor processingequipment. As is explained in further detail below, the device layer 12is now readily used for conventional semiconductor processing to makeany manner of electronic circuits. The direct wafer bonding method 10 ofthe present invention allows a much higher resistivity device layer thanconventional epitaxy or diffusion due to the lower overalldiffusion-time product (DT product) to produce a bonded structurecompared to epitaxy.

Referring now to FIG. 2, a graph of a DWB doping profile according tothe present invention shows the doping density of a handle wafer, thedoping density of a device wafer, and the abrupt transition between thetwo doping densities in an interface region. In the example of FIG. 2, aPiN diode doping profile is shown. The doping profile will of course bedifferent for different semiconductor devices. The example of FIG. 2 ispresented to clearly illustrate the abrupt transition between the devicelayer and the handle wafer. Thus, the doping density value 20 is shownto be about 1E19 ions/cm³ in a p+ anode, down to a depth of about twomicrons as shown. From two to about 13 microns, the doping density value20 is about 1E12 ions/cm³ (2,000 Ohm-cm). The device layer is shown tobe of the n− doping type in FIG. 2. From a depth of about 13 microns to19 microns, a transition region is shown. A first boundary of thetransition region is shown at 22, which is physically within the devicelayer 12. The increase in the transition region doping value 20 fromabout 1E12 ions/cm³ to 1E19 ions/cm³ is due to out diffusion from thehighly doped handle wafer 14. Finally, the bond interface between thetwo wafer layers is shown at 24 at a depth of about 19 microns. Thehandle layer forms the n+ cathode of the PiN diode, with a dopingdensity of about 1E19 ions/cm³.

Ideally, a one-sided step junction is formed with greater than sixorders of magnitude difference in carrier concentration within a depthof about six microns. (One order of magnitude change in carrierconcentration per micron of depth.) The handle wafer resistivity is madeextremely low, less than 0.01 Ohm-cm being preferred.

Referring now to FIG. 3, an illustration of a bonded wafer 30 accordingto the present invention, and the corresponding perpendicular (B-B′) andparallel (A-A′) conductivity profiles thereof. In the top portion ofFIG. 3, a bonded wafer 30 is shown having a device layer 32 and a handlewafer 34. A mask layer 36 is patterned such that dopants 38 can beimplanted. In the middle portion of FIG. 3, a first cross section A-A′is parallel to the surface of the device layer 32, with the crosssection being taken through the middle of the device layer. Note that,due to the dopant implantation, the doping profile through A-A′ canvary, for example, from 1E13 ions/cm³ in the non-implanted regions to1E16 ions/cm³ in the implanted regions. In the bottom portion of FIG. 3,a second cross section B-B′ is perpendicular to the surface of thedevice layer 34, with the cross section being taken through the devicelayer 32 and the handle wafer layer 34. Note that, since the crosssection B-B′ extends through the device layer 34 in a non-implantedregion, and into the heavily doped handle wafer, the doping densitychanges from about 1E13 ions/cm³ to 1E19 ions/cm³, with an abrupttransition region.

Referring now to FIG. 4, an illustration of a bonded wafer 40 is shownaccording to the present invention including an integrated circuit (aninverter) fabricated in the device wafer 42. Although a simple inverteris shown in FIG. 4, it will be appreciated by those skilled in the artthat many other devices can be built in the device wafer using similardoped regions, isolation regions, and gate layers as shown. In FIG. 4, ahandle wafer 44 is shown having a p++ doping profile. A p− device layer42 is also shown. In the device layer 42, isolation regions, typicallyoxide regions, are shown. Doped regions 48 (n+) and 50 (p+) are alsoshown. Patterned gate oxide regions 54, and metal gate contacts 52 arealso shown. As is known in the art, the doped regions are coupledtogether to form a VDD contact 56, an input 60, an output 62, and a VSScontact 58. The exact structure shown in FIG. 4 is illustrative of onetype of circuit that can be fabricated in the device layer 42 of thebonded wafer 40 according to the present invention.

In a first embodiment of the present invention, a wafer structureincludes a first region (from the device wafer) 12 having at least onemajor surface, a thickness and a conductivity profile of a firstconductivity type substantially perpendicular to the at least one majorsurface, a second region (from the handle wafer) 14 having a thicknessand a second conductivity profile of the first conductivity type of thefirst region 12, such second conductivity profile being substantiallydifferent than the conductivity profile of the first region 12, suchthat the second region 14 is in electrical contact with the first regionopposite the major surface of the first region, an interface region 13being formed between the first region 12 and the second region 13, andimpurity sites placed in at least one of the first region 12, the secondregion 14, and the interface region 13, such impurity sites beingsubstantially electrically inactive over a predetermined temperaturerange (−75° C. to +200° C. degrees), wherein the conductivity profile ofthe first region 12 transitions abruptly (greater than one order ormagnitude per one micron) to the conductivity profile of the secondregion 14 within the interface region 13.

The first region 12, the second region 14, and the interface region 13each comprises a semiconductor material such as silicon, galliumarsenide, or germanium. The impurity sites are selected from the groupof isotopes of germanium, silicon, carbon, fluorine, sulfur, chlorine,nitrogen, or defects selected from the group of lattice vacancies,interstitial defects, Frenkel defect pairs, crystal dislocations, orother defects such as strained layers of the semiconductor lattice, or acombination of said isotopes and defects. The impurity sites areincorporated into at least one of the first region 12, the second region14, and the interface region 13 by way of ion implantation, diffusionfrom a solid, liquid, or gaseous source, during growth of an epitaxiallayer, bombardment by heavy ions, neutrons, protons, or electrons, or acombination thereof. The impurity sites can be introduced into theseparate device layer 12, the separate handle wafer 14, and also thecombination of the two wafers once bonded, and ground to the desiredthickness, which would also then include the interface region 13. One,two, or all three of these introduction steps may be performed asdesired for a specific application.

The introduction of defects using neutron bombardment is especiallyproblematic due to the undesirable side effects of displacement damageand total ionizing dose degradation that are unavoidable with manyneutron sources such as nuclear reactors. However, the present inventionsolves this problem by using a high energy, nearly monoenergetic sourceof neutrons with an energy in the range of 10 to 50 MeV, with 32 MeVbeing the preferred energy. Furthermore, the total fluence is optimizedto provide the optimum defect concentration in the first region 12, andthe interface region 13. Neutron fluences range from about 2E12neutrons/cm² to 2E14 neutron/cm², with 2E13 neutrons/cm² beingpreferred. The neutron source used ideally has low emission of ionizingelectromagnetic radiation, and is ideally a spallation source, whereinneutrons are generated by nuclear reaction in a material caused bybombardment with a charged particle beam, with a proton ion beam beingthe preferred charged particle beam.

The device layer 12 is now well suited for making electronic circuitsthat are substantially immune to SEL, because the charge collectionregion for such events has been limited to device layer 12, and theminority carrier lifetime contributing to the gain of the parasitic SCRdevices that cause SEL has been lowered substantially due to theincorporation of impurity sites as defined above. In reduced thicknessof the device layer 12 enabled by the lower DT product, also reduces theoverall SEU error rate of an IC in a particular radiation environment bylimiting the amount of collected charge. The incorporation of impuritysites in the first Region 12 is also designed to inhibit the diffusionof certain dopants at the silicon:silicon-dioxide interface, thuspinning the surface potential at such interfaces such that theinevitable TID-induced trapping of positive charge in the overlyingdielectric layers, including silicon dioxide, does not cause the siliconsurface to change its surface potential substantially, in particularpreventing the silicon surface potential from inverting from oneconductivity type to the opposite conductivity type during or afterexposure to ionizing radiation. The incorporation of impurity sites, asdefined above, into the interface region 13 also acts to retard thediffusion of dopants from the second region 14, thus preventing anincrease in the conductivity of the first region 12. Such increase inthe conductivity of first region 12 is deleterious to the electricalperformance of the IC, causing an increase in reverse bias junctionleakage of the IC, and in sever cases an unacceptable threshold voltageshift in the MOS transistors of the IC, and/or a decrease in themobility of the channel regions of such MOS transistors of the IC.Impurity sites incorporated into the second region 14 are also effectivein retarding diffusion of dopants from the second region 14 into thefirst region 12 through the interface region 13.

Typically, the structure of the present invention is formed whereby thefirst region 12 and the second region are formed of two distinctsubstrates from two different wafers that are subsequently bondedtogether and then ground down to a desired final thickness. Advances inepitaxy or diffusion, however, may permit the abrupt change in carrierconcentration (greater than one order of magnitude for one micron ofdepth) to be accomplished in a single wafer. The present inventioncontemplates a single substrate embodiment fabricated using theseadvanced epitaxy and diffusion techniques. Organic chemical vapordeposition (MOCVD) could also be used to form the device layer 12.

The structure of the present invention contemplates that the firstconductivity type is either p-type or n-type. At least one of the firstregion 12, the second region, and the interface region incorporates atleast one dopant. The wafers may be separately doped and bonded.Additional doping can be accomplished once the bonding step has beencompleted. The dopant is selected from the group of isotopes includingboron, phosphorous, arsenic, antimony, aluminum, gallium, or acombination thereof. The incorporation of the at least one dopant intoat least one of the first region 12, the second region 14, and theinterface region 13 is accomplished by way of ion implantation,diffusion from a solid, liquid, or gaseous source, during growth of anepitaxial layer, or a combination thereof. Again, this can be done withseparate wafers 12 and 14, but additional processing can be done oncethe wafers have been bonded together.

In the present invention, the transition of the conductivity profilethat occurs abruptly in the interface region 13 is at least one order ofmagnitude of conductivity per micron of thickness. The thickness of thefirst region 12 is made sufficient for manufacturing at least one activesemiconductor device therein. Typically, this thickness will be aboutone to ten microns, or slightly deeper if desired for a particularapplication. The combined thicknesses of the first region 12, the secondregion 14, and the interface region 13 is made to conform with standardsfor ease of processing on semiconductor manufacturing equipment. Forexample, a combined thickness of about 725 microns for presentmanufacturing equipment is deemed to be desirable.

The structure of the present invention can include at least one activesemiconductor device that does not latch-up during irradiation withprotons, neutrons, heavy ions, or bursts of gamma rays, electrons, orother particles or waves that cause ionization in the at least oneactive semiconductor device. As mentioned above, the semiconductordevice will be inherently immune to latch-up because the chargecollection region has been limited without compromising the conductivityof device region 12, and because the gains of the parasitic SCR devicesthat are inherent in CMOS IC's have been sufficiently reduced to thepoint where such deposited charge cannot induce a latch-up in suchparasitic SCR devices. The heavy ions comprise an ion beam with aneffective linear energy transfer in silicon greater than or equal to 0.1MeV cm²/mg. The protons comprise a beam of protons with an effectivelinear energy transfer in silicon greater than zero but less than orequal to 1 MeV cm²/mg. The irradiation comprises a neutron environmentwith an energy distribution within the range of 0.1 MeV to 100 MeV.Gamma ray and/or electron bursts comprise events with durations from 1to 1,000 nanoseconds with dose rates from 1E4 to 1E12 rad(Si)/second.The thickness of the said first region 12 is no greater than thatcalculated to limit the charge collection arising from the heavy ionirradiation. Because the energy loss per unit length varies fordifferent ions the maximum LET value for which latch-up immunity isachieved can be varied by varying the charge collection volume. In otherwords, the latch-up immunity is determined by the process parametersselected.

The structure of the present invention includes at least one circuitmade up of a plurality of active semiconductor devices. The circuit cancomprise at least one circuit selected from the group of analog todigital converter, digital to analog converter, voltage regulator,voltage reference, voltage monitor, operational amplifier, comparator,microprocessor, microcontroller, static random access memory, dynamicrandom access memory, rf transmitters, if demodulators, system clock,sensor interface, or analog filter. The circuit is typicallyincorporated in a system selected from the group of satellite telemetrycontrol, satellite attitude control, satellite sensors, satellitecommunications, satellite reaction wheel, satellite antenna. In turn,the system is incorporated into at least one of the group of launchvehicle, orbiter, satellite, missile, manned spacecraft, and othervehicles intended for high altitude (altitudes greater than 50,000 feet)operation. However the achieved latch-up immunity can also be used indeveloping parts used in high radiation environments such as reactors,medical radiation facilities and research facilities such asaccelerators.

In the structure of the present invention, the first region 12 can bemade either co-extensive or not co-extensive with the second region 14,as shown in the wafer structure 600 of FIG. 6. In one case the regionsof the first region 12 that are not co-extensive with the second region14 act as isolation regions between, for example, bipolar junctiontransistors. Two interface regions 13 can also be seen in the waferstructure 600 of FIG. 6.

In the structure of the present invention, at least one additionalregion having a thickness and a conductivity profile of the firstconductivity type of the first region may be included, and at least oneadditional interface region between the first region and the at leastone additional region may also be included as shown in the waferstructure 500 of FIG. 5. Such additional interface region may comprisean epitaxial wafer used as one of two wafers bonded together, theepitaxial wafer already incorporating an interface region.Alternatively, a wafer structures with multiple interface regions can beconstructed through multiple wafer bonding steps, each bonding stepbeing followed with grinding and polishing steps such that the distancebetween the multiple interface regions can be controlled. The waferstructure 500 of FIG. 5 shows two first regions 12, two interfaceregions 13, and a second region 14.

In the first embodiment of the invention, the device wafer 12 and thehandle wafer were of the same conductivity type (both either p-type orn-type) but with markedly different doping profiles as was shown in FIG.2. In a second embodiment of the invention, the device wafer 12 and thehandle wafer 14 are of the opposite conductivity type (p-type andn-type, or n-type and p-type, respectively). A typical bonded waferaccording to the second embodiment of the invention is an n-type handlewafer 14 having a resistivity of less than 0.01 Ohm-cm, and a p-typedevice wafer 12 having a resistivity of 100 Ohm-cm.

According to a second embodiment of the invention, a wafer structureincludes a first region 12 having at least one major surface, athickness, and a conductivity profile of a first conductivity typesubstantially perpendicular to said at least one major surface, a secondregion 14 having a thickness, and a second conductivity profile of asecond conductivity type opposite to that of said first region, suchthat said second region is in electrical contact with said first regionopposite the major surface of said first region, an interface region 13formed between said first region and said second region, and impuritysites placed in at least one of said first region, said second region,and said interface region, such impurity sites being substantiallyelectrically inactive over a temperature range, wherein the conductivityprofile of said first region transitions abruptly to the conductivityprofile of said second region within the interface region. The primarydifference between this embodiment and the first is the direct formationof a junction and the use of a different conductivity type substrate.

In a third embodiment of the present invention, the parallelconductivity profile of the device layer 32, best shown in FIG. 3 isemphasized. In the third embodiment of the present invention, the devicelayer 32 has a parallel conductivity profile due to implantation 38 inunmasked areas of the surface thereof. This is particularly shown incross section A-A′ of FIG. 3, as previously described. In the thirdembodiment of the present invention, the device layer 32 and the handlewafer 34 are of the same conductivity type, either p-type or n-type.According to the third embodiment of the present invention, a waferstructure 30 includes a first region 32 having at least one majorsurface, a thickness, and a conductivity profile of a first conductivitytype substantially parallel to said at least one major surface, a secondregion 34 having a thickness, and a second conductivity profile of thefirst conductivity type of said first region, such second conductivityprofile being substantially different than the conductivity profile ofsaid first region, such that said second region is in electrical contactwith said first region opposite the major surface of said first region,an interface region 33 formed between said first region and said secondregion, and impurity sites placed in at least one of said first region,said second region, and said interface region, such impurity sites beingsubstantially electrically inactive over a temperature range, whereinthe conductivity profile of said first region transitions abruptly tothe conductivity profile of said second region within the interfaceregion.

In a fourth embodiment of the present invention, the parallelconductivity profile of the device layer 32, best shown in FIG. 3 isagain emphasized. In the third embodiment of the present invention, thedevice layer 32 has a parallel conductivity profile due to implantation38 in unmasked areas of the surface thereof. This is particularly shownin cross section A-A′ of FIG. 3, as previously described. In the thirdembodiment of the present invention, the device layer 32 and the handlewafer 34 are of the opposite conductivity type, either p-type andn-type, or n-type and p-type, respectively. According to the fourthembodiment of the present invention, a wafer structure 30 includes afirst region 32 having at least one major surface, a thickness, and aconductivity profile of a first conductivity type substantially parallelto said at least one major surface, a second region 34 having athickness, and a second conductivity profile of a second conductivitytype opposite to that of said first region, such that said second regionis in electrical contact with said first region opposite the majorsurface of said first region, an interface region 33 formed between saidfirst region and said second region, and impurity sites placed in atleast one of said first region, said second region, and said interfaceregion, such impurity sites being substantially electrically inactiveover a temperature range, wherein the conductivity profile of said firstregion transitions abruptly to the conductivity profile of said secondregion within the interface region.

In a fifth embodiment of the invention, the integrated circuit nature ofthe wafer structure of the present invention is emphasized. Theintegrated circuit nature of the wafer structure is best seen in FIG. 4,previously described. A radiation hardened integrated circuit 40according to the fifth embodiment of the present invention includes aplurality of semiconductor devices (represented by doped regions 48 and50, isolation regions 46, and gate contacts 52) formed in a waferstructure, said wafer structure including a first region 42 having atleast one major surface, a thickness, and a conductivity profile of afirst conductivity type substantially perpendicular to said at least onemajor surface, a second region 44 having a thickness, and a secondconductivity profile of the first conductivity type of said firstregion, such second conductivity profile being substantially differentthan the conductivity profile of said first region, such that saidsecond region is in electrical contact with said first region oppositethe major surface of said first region, an interface region 43 formedbetween said first region and said second region, and impurity sitesplaced in at least one of said first region, said second region, andsaid interface region, such impurity sites being substantiallyelectrically inactive over a temperature range, wherein the conductivityprofile of said first region transitions abruptly to the conductivityprofile of said second region within the interface region.

In a sixth embodiment of the invention, forming of the impurity sites byhigh energy bombardment is emphasized. The following description willgenerally refer again to FIG. 1, and also to additional FIGS. 7 and 8.

Thus, in FIG. 1, a wafer structure 10 includes a first region 12 havingat least one major surface, a thickness, and a conductivity profile of afirst conductivity type substantially perpendicular to said at least onemajor surface, a second region 14 having a thickness, and a secondconductivity profile of the first conductivity type of said firstregion, such second conductivity profile being substantially differentthan the conductivity profile of said first region, such that saidsecond region is in electrical contact with said first region oppositethe major surface of said first region, an interface region 13 formedbetween said first region and said second region, and impurity sitesplaced in at least one of said first region, said second region, andsaid interface region, by bombardment of high energy monoenergeticneutron irradiation. At least one of said first region 12, said secondregion 14, and said interface region 13 includes a semiconductormaterial, for example silicon, and the completed material has beentreated with high-energy monoenergetic neutron irradiation. Themonoenergetic neutron energy can be between 1 MeV and 14-MeV and can becreated by a Deuterium-Deuterium (D-D) reaction. The first region 12 andthe second region 14 can be formed by two distinct substrates and eitherthe first or second region, or both are bombarded by the high-energy,monoenergetic neutrons. The formation can be bonding and either thefirst or second region, 12 or 14, or both are bombarded by high-energymonoenergetic neutrons before or after processing. The first region 12and the second region 14 can also be formed in a single substrate thathas been altered by high-energy monoenergetic neutron irradiation toprovide for said first region, said second region, and said interfaceregion. The first region 12 can include an epitaxial layer depositedupon said second region 14 and the completed material has been treatedwith high-energy monoenergetic neutron irradiation. The first region 12can be a layer deposited upon the second region 14 by metal organicchemical vapor deposition (MOCVD) and the completed material has beentreated with high-energy monoenergetic neutron irradiation. The firstconductivity type is selected from the group of p-type or n-type. Theconductivity profile of the at least one of said first region 12, saidsecond region 14, and said interface region 13 incorporates at least onedopant and the structure is treated with high-energy monoenergeticneutron irradiation. The dopant is selected from the group consisting ofisotopes of boron, phosphorous, arsenic, antimony, aluminum, gallium, ora combination thereof. The incorporation of at least one dopant into atleast one of said first region 12, said second region 14, and saidinterface region 13 can be accomplished by way of ion implantation,diffusion from a solid, liquid, or gaseous source, during growth of anepitaxial layer, or a combination thereof. The transition of aconductivity profile that occurs abruptly in the interface region is atleast one order of magnitude of conductivity per micron of thickness.The first region thickness is sufficient for manufacturing at least oneactive semiconductor device therein, which device is ideallysubstantially immune to latch-up during irradiation with protons,neutrons, or heavy ions. The heavy ions can comprise a beam of heavyions with an effective linear energy transfer in silicon greater than orequal to 0.1 MeV cm²/mg. The protons can comprise a beam of protons withan effective linear energy transfer in silicon greater than zero butless than or equal to 1 MeV cm²/mg. The irradiation can comprise aneutron environment with an energy distribution within the range of 0.1MeV to 100 MeV. The thickness of said first region 12 can be no greaterthan that calculated to truncate the charge collection of said heavy ionirradiation. The finished structure shown in FIG. 1 can include at leastone circuit made up of a plurality of active semiconductor devices. Thecircuit can be selected from the group of analog to digital converter,digital to analog converter, voltage regulator, voltage reference,voltage monitor, operational amplifier, comparator, microprocessor,microcontroller, static random access memory, dynamic random accessmemory, RF transmitters, IF demodulators, system clock, sensorinterface, or analog filter. The circuit is ideally incorporated in asystem selected from the group of satellite telemetry control, satelliteattitude control, satellite sensors, satellite communications, satellitereaction wheel, or satellite antenna. The system is incorporated into atleast one of the group of launch vehicle, orbiter, satellite, missile,manned spacecraft, and other vehicles intended for high altitudeoperation. The first region 12 can be co-extensive with the secondregion 14. Alternatively, the first region 12 is not co-extensive withthe second region 14. The structure of the present invention cancomprise at least one additional region having a thickness and aconductivity profile of the first conductivity type of said firstregion, and at least one additional interface region between said firstregion and said at least one additional region, as previously discussed.The combined thicknesses of said first region 12, said second region 14,and said interface region 13 conforms with a predetermined standard forprocessing on semiconductor manufacturing equipment.

A method of forming a wafer structure 10 according to the presentinvention includes forming a first region 12 in a device wafer having atleast one major surface, a thickness, and a conductivity profile of afirst conductivity type substantially perpendicular to said at least onemajor surface, forming a second region 12 in a handle wafer having athickness, and a second conductivity profile of the first conductivitytype of said first region, such second conductivity profile beingsubstantially different than the conductivity profile of said firstregion, such that said second region is in electrical contact with saidfirst region opposite the major surface of said first region, bondingtogether said device and handle wafers, forming an interface region 13formed between said first region 12 and said second region 14, andplacing impurity sites in at least one of said first region, said secondregion, and said interface region by high-energy monoenergetic neutronbombardments. The method can include grinding said bonded device andhandle wafers, the structure having been treated with high-energymonoenergetic neutron bombardments. The method can also include treatingthe wafer structure with a high-energy monoenergetic neutron sourceobtained from a deuterium-tritium reaction. The method can also includetreating the wafer structure with a high-energy monoenergetic neutronsource obtained from a deuterium-deuterium reaction. The method can alsoinclude treating the wafer structure with a high-energy monoenergeticneutron source obtained from a secondary reaction of charged particles.The method can also include treating the wafer structure with ahigh-energy monoenergetic neutron source obtained from a secondaryreaction of protons.

A high-energy monoenergetic neutron source according to the presentinvention is further described below with respect to FIGS. 7 and 8.

A monoenergetic neutron source suitable for cost-effective semiconductortreatment is a pumped, drift tube accelerator which may be used as apositive ion or an electron accelerator. For neutron generation, thesource is configured as a deuteron accelerator using either a tritium-or deuterium-impregnated metal film target. Neutron yields up to 2×10¹¹n/s may be achieved using a tritiated titanium target, with yields of upto 1×10⁹ n/s using a deuterated target.

The neutrons are created by deuterium ions accelerating into a tritiumor deuterium target yield the following energies:

D+T→n+ ⁴He E_(r)=14.2 MeV

D+D→n+ ³He E_(n)=2.5 MeV

Neutrons produced from the D-T reaction are emitted isotropically(uniformly in all directions) from the target. Neutron emission from theD-D reaction is slightly peaked in the forward (along the axis of theion beam) direction. In both cases, the He nucleus (alpha particle) isemitted in the exact opposite direction from the neutron.

Neutron fluence at 14 MeV is determined using radioactivationtechniques, employing the standard method described in ASTM publicationE 496-96. Where possible, niobium activation foils are placed on thesamples during exposure. After completion of the irradiation, activitylevels of these foils are read using an NaI gamma-ray spectrometer.Neutron fluence determination at 2.5 MeV is similar, except that uraniumfoils are used.

FIG. 7 shows a two-dimensional map of 14 MeV neutron flux as a functionof distance from the source, for a source output of 1×10¹¹ n/s. Thecontours deviate from a purely 1/r² dependence at close range, becauseof the finite size of the source.

An irradiation cell design, layout, and operation is shown in furtherdetail in FIG. 8. A plan view of the neutron irradiation test cell 800is shown in FIG. 8. The cell has interior dimensions of approximately 5ft by 8 ft, with walls formed of solid concrete block. Wall thickness isapproximately 80 inches. Access to the test cell is provided by adouble-wall door arrangement, consisting of a 48-inch-thick woodenstructure filled with polyethylene beads and a 16-inch-thick concretedoor. Both doors roll on tracks set on the concrete floor. An outer door802 and inner door 804 are shown in FIG. 8. Also shown in FIG. 8 arecable conduit 806, bench 808, neutron source control console 810, HVpower supplies 812, and neutron source 814.

Sample holders fabricated from aluminum or other suitable structuralmaterial are situated in the inside of the test cell. It should be notedthat whole semiconductor wafers containing completed integratedcircuits, individual completed integrated circuits in die form, and/orpackaged integrated circuits may be irradiated using the appropriatesample holders.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the invention withoutdeparting from the spirit or scope of the invention. Thus, it isintended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

We claim:
 1. A method of forming a wafer structure comprising: forming afirst region in a device wafer having at least one major surface, athickness, and a conductivity profile of a first conductivity typesubstantially perpendicular to said at least one major surface; forminga second region in a handle wafer having a thickness, and a secondconductivity profile of the first conductivity type of said firstregion, such second conductivity profile being substantially differentthan the conductivity profile of said first region, such that saidsecond region is in electrical contact with said first region oppositethe major surface of said first region; bonding together said device andhandle wafers; forming an interface region formed between said firstregion and said second region; and placing impurity sites in at leastone of said first region, said second region, and said interface region,such impurity sites being substantially electrically inactive over atemperature range, wherein the conductivity profile of said first regiontransitions abruptly to the conductivity profile of said second regionwithin the interface region.
 2. The method of claim 1 further comprisingthinning one or both of said bonded device and handle wafers.
 3. Themethod of claim 2 wherein thinning comprises grinding the device wafer.4. The method of claim 2 wherein thinning comprises grinding the handlewafer.
 5. The method of claim 2 wherein thinning comprises grinding to apredetermined thickness.
 6. The method of claim 2 wherein thinningcomprises polishing.